全新java初学者实践教程10(java SE5.0版)
`1P|<VbZ jdk5的集合类
0sU*3 r? <$ssU{5 sM MtU@<x x5MS#c!7 上次课我们学过了数组,知道它只是一组数(或是对象),但是有些自己的特性。在[font="Times]java里还有一类东西与数组类似,也是有着特性的一组数[font="Times](或是对象[font="Times]),叫做集合类。
czIAx1R9 我们上节课讲到了,数组的长度在创建时已经确定了,但是有时候我们事先根本不知道长度是多少啊,比如我们做电子商务网站时,有个购物车程序。你总不能用数组规定,人家只能买[font="Times]5样东西吧。你就是把长度定为[font="Times]10000也不行,万一遇上个特别有钱的呢!呵呵,这只是开玩笑的。我们会使用集合类解决这个问题。
[m{sl(Q [font="Times] 集合类是放在[font="Times]java.util.*;这个包里。集合类存放的都是对象的引用,而非对象本身,为了说起来方便些,我们称集合中的对象就是指集合中对象的引用([font="Times]reference)。引用的概念大家不会忘了吧,在前边我们讲
数据类型时讲的。
N,K/Ya)1 wH!$TAZ:Yw [font="Times] 集合类型主要有[font="Times]3种:[font="Times]set(集)、[font="Times]list(列表)、[font="Times]map(映射[font="Times])和[font="Times]Queue(队列)。[font="Times]//队列为[font="Times]jdk5中的加上的[font="Times]
j24 3oD mrRid}2 [font="Times](1) Set
66F?exr 集([font="Times]set)是最简单的一种集合,它的对象不按特定方式排序,只是简单的把对象加入集合中,就像往口袋里放东西。对集中成员的访问和操作是通过集中对象的引用进行的,所以集中不能有重复对象。我们知道数学上的集合也是[font="Times]Set这个,集合里面一定是没有重复的元素的。
5b/ ~]v -t S\ ([font="Times]2)[font="Times]List
:,JjN& 列表([font="Times]List)的主要特征是其对象以线性方式存储,没有特定顺序,只有一个开头和一个结尾,当然,它与根本没有顺序的[font="Times]Set是不同的。它是链表嘛,一条链肯定有顺序这个顺序就不一定了。
BVeMV4 `dcz9 * _b%) [font="Times] ([font="Times]3)[font="Times]Map
W;=Ae~ [font="Times] 映射([font="Times]Map),这个在[font="Times]java里不是地图的意思,其实地图也是映射哈。它里面的东西是键-值对([font="Times]key-value)出现的,键值对是什么呢?举个例子,比如我们查字典,用部首查字法。目录那个字就是键,这个字的解释就是值。键和值成对出现。这样说可以理解吧。这也是很常用的数据结构哦。
/;(ji?wN Ur]$@N #0T/^ # [font="Times] ([font="Times]4)[font="Times]Queue
hT 1JEu [font="Times] 在[font="Times]jdk5.0以前,通常的实现方式是使用[font="Times]java.util.List集合来模仿[font="Times]Queue。[font="Times]Queue的概念通过把对象添加(称为[font="Times]enqueuing的操作)到[font="Times]List的尾部(即[font="Times]Queue的后部)并通过从[font="Times]List的头部(即[font="Times]Queue的前部)提取对象而从[font="Times] List中移除(称为[font="Times]dequeuing的操作)来模拟。你需要执行先进先出的动作时可以直接使用[font="Times]Queue接口就可以了。
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0\r [font="Times] 这[font="Times]4个东西,有时候功能还不太完善,需要有些子类继承它的特性。[font="Times]Set的子接口有[font="Times]TreeSet,SortedSet,[font="Times]List的有[font="Times]ArrayList等,[font="Times]Map里有[font="Times]HashMap,HashTable等,[font="Times]Queue里面有[font="Times]BlockingQueue等。我们来看看例子吧:
6Jq[]l"v -_Z 4)"k %gO/mj3* 实践:[font="Times] Set举例
5\z<xpJ [font="Times] import java.util.*;
8>[g/%W [font="Times]public class SetExample {
YX-~?Pl [font="Times] public static void main(String[] args) {
PlH~um[J [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
-!_8>r;Q4 [font="Times] set.add("one");
Kw`CN [font="Times] set.add("second");
BZ:tVfg. [font="Times] set.add("3rd");
131(0nl)=I [font="Times] set.add(new Integer(4));
xrvM}Il [font="Times] set.add(new Float( 5.0F ));
1Zn8CmE V [font="Times] set.add("second");
R`c[?U [font="Times] set.add(new Integer(4));
DNq(\@x[! [font="Times] System.out.println(set);
k o[w#j [font="Times] }}
u*Xp%vNe &
V>rq'~; [font="Times]List举例:
1}a4AGAp [font="Times] import java.util.*;
(&eF E ;c [font="Times]public class ListExample {
t}_ #N'` [font="Times] public static void main(String[] args) {
*'{-!Y [font="Times] List list = new ArrayList();
3<W%z]k@M [font="Times] list.add("one");
:6lv X$ [font="Times] list.add("second");
iiQn/% [font="Times] list.add("3rd");
-JgNujt#9 [font="Times] list.add(new Integer(4));
M]r?m@) [font="Times] list.add(new Float( 5.0F ));
=w+8q1!o [font="Times] list.add("second");
:K^J bQ [font="Times] list.add(new Integer(4));
V2}\]x'1 [font="Times] System.out.println(list);
PhC3F4 [font="Times] }}
:CE4<
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U&WEe`XM [font="Times]Map举例
-%"PqA/1zj [font="Times]import java.util.Map;
V_gKl;Kfe8 [font="Times]import java.util.HashMap;
7C7.}U [font="Times]import java.util.Iterator;
At:8+S<?A [font="Times]import java.io.FileReader;
?'P}ZC8P [font="Times]
<r:AJ; [font="Times]public class MapExample {
B%;MGb o [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
c$V5E t [font="Times] Map word_count_map = new HashMap();
[y@*vQw [font="Times] FileReader reader = new FileReader(args[0]);
a,vS{434J [font="Times] Iterator words = new WordStreamIterator(reader);
iv$YUM+ [font="Times]
+v;z^+ [font="Times] while ( words.hasNext() ) {
;WSW&2 [font="Times] String word = (String) words.next();
&t9V [font="Times] String word_lowercase = word.toLowerCase();
=p'+kS+ [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
JnsJ]_< [font="Times]if ( frequency == null ) {
r+Ki`HD% [font="Times] frequency = new Integer(1);
t>cGfA [font="Times] } else {
:Mu*E5 [font="Times] int value = frequency.intValue();
ElQ?|HsQ6p [font="Times] frequency = new Integer(value + 1);}
7v%c. [font="Times] word_count_map.put(word_lowercase, frequency);
\_1a#|97e [font="Times] }
Io.RT+slB [font="Times] System.out.println(word_count_map);
|2q3spd [font="Times] }}
[oTe8^@[ h )
Wp {zAI-?#*u [font="Times]Queue举例:
DRQx5fgL [font="Times]import java.io.IOException;
RHC ZP [font="Times]import java.io.PrintStream;
xrkl)7; [font="Times]import java.util.LinkedList;
GjhTF| [font="Times]import java.util.Queue;
Kn~Rck|
] [font="Times]
_Ub
`\ytx [font="Times]public class QueueTester {
!e|\1v'0 [font="Times] public Queue q; //发现了一个奇怪的语法,这个尖括号是泛型声明
!B3TLeh [font="Times] public QueueTester() {q = new LinkedList();}
R (~wSL*R> [font="Times]public void testFIFO(PrintStream out) throws IOException {
H\S)a FY[ [font="Times] q.add("First");
lDYgtUKG [font="Times] q.add("Second");
[7v|bd [font="Times] q.add("Third");
5^ Qa8yA>7 [font="Times] Object o;
!y_{mE?V( [font="Times] while ((o = q.poll()) != null) {
|Ghk8 WA [font="Times] out.println(o);}}
Q6Gw!!Z5EA [font="Times] public static void main(String[] args) {
zi-_ l [font="Times] QueueTester tester = new QueueTester();
#Lhv=0op [font="Times] try { tester.testFIFO(System.out);
J3 _aHI [font="Times] } catch (IOException e) {
u;_~{VJ- [font="Times] e.printStackTrace(); } }}
uNzc,OH p:4jY|q h+[6i{ 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
O_:l;D#i 总结:
_nbr%PD, 刚才我们看了上述例子了,对集合类有了一个初步的认识,它们跟数组的区别不只是长度问题,在集合类里面放进去的类型可以是任意的。不像数组,数组里面的类型是一样的。这样的话,对于集合类来说即使好事,也是坏事。因为你不考虑类型可以随意的放,但是你放进去什么就不知道了不容易找。
aZA``#p+ 还有啊,什么叫泛型声明啊?我们下次课就告诉你。这可是[font="Times]jdk5的酷炫之处哦。